OCTANE AND ALTITUDE
NATURALLY ASPIRATED ENGINES
Atmospheric pressure has a significant impact on the in-cylinder pressures of naturally aspirated engines during WOT. As Johannesburg, for example, is roughly 1 700 metres above sea level, the atmospheric pressure is generally 17% lower than at the coast, explaining the drop in engine performance. This fact lowers the tendency of the engine to knock, allowing a lower-octane fuel to be used with no loss in performance compared with a higher-octane fuel because the additional benefit cannot be realised.
This was confirmed in a study by Professor Andy Yates, who conducted actual WOT performance testing on a fleet of 18 vehicles in Gauteng using varying octane-rated petrol. He found the following:
The top graph depicts the probability of maximum performance gain by octane number at altitude. Most manufacturers calibrate their petrol engines to run optimally on 95 octane at the coast, although some gains are still possible. Interestingly, 87-octane petrol appears to satisfy most engines’ maximum performance potential at altitude and is confirmed by the second graph showing no benefit at altitude to use an octane rating higher than 91. Studies show, with each increase of 300 metres in altitude, drops in octane rating of between 1,0 and 1,8 can be safely used with no negative effect on performance from a Mbt-timing point of view.
Basis: SAE paper 2003-01-2012, Fleet Tests to Determine the Octane Response at Di erent Altitudes for Vehicles Equipped with Knock Sensors